Does it waste more gas to keep your car running in park for a while, or to turn it off and then back on?

Answers

Yes, it is almost always more wasteful to idle. Ars Cal mentioned the 6 second figure and although I have heard 10 seconds, the difference is negligible. If you are stopping, the odds are good that it will be for over 10 seconds. As long as it is safe, be sure to turn the car off when you are waiting.